1. Field of the Invention
The present invention relates to submersible pumps, and more particularly relates to a submersible pump employing a scavenger system to protect the pump motor from corrosive agents present in the well fluid.
2. Background
The fluid in many producing oil and/or gas wells is elevated to the surface of the ground by the action of a pumping unit or a pumping apparatus installed in the lower portion of the well bore. In recent times there has been increased activity in the drilling of well bores to great depths. In addition, the use of water flooding for additional fluid recovery in oil fields, wherein the production of the subsurface fluid has been somewhat depleted, is commonly practiced. This water flooding has produced a considerable quantity of downhole fluid in the well bore. As a result it has become necessary to install downhole pumps within the fluid contained within the well bore.
Electrical submersible pumps (ESPs) are used for lifting fluid from bore holes. In operation, an electrical submersible pump's motor and pump are placed below the fluid surface in the bore hole. The well fluid often contains corrosive compounds such as brine water, CO.sub.2, and H.sub.2 S that can shorten the run life of an ESP since the ESPs are directly exposed to the well fluid. Attempts to solve the problem of corrosion include the development of corrosion resistant units. The motors in these corrosion resistant units generally use seals and barriers to exclude the corrosive agents from the internal mechanisms of the ESP.
Electrical submersible pump motors have special problems that make the motors difficult to protect from corrosion. For example, the motor is filled with fluid, typically a dielectric oil, for lubrication and cooling. As the motor operates, the oil heats up and expands. When the unit is shut off, the fluid cools and contracts. This expanding and contracting, or "breathing", of the motor fluid makes it necessary to provide an expansion mechanism to accommodate the heated fluid. This expansion mechanism usually uses baffles creating tortuous flow path for the oil or a barrier that is either an elastomeric bladder or a direct interface between the well fluid and the motor fluid. A problem is that corrosive agents can cross the tortuous flow path or cross or penetrate the barrier and attack the internal components of the motor. The design of an electric motor for a submersible pump requires the use of copper for the magnetic coils and other metals and materials that can be corroded or degraded by the presence of corrosive compounds. This phenomena becomes severe in higher temperature wells.